Recently, with the tendencies of miniaturization and high speed of electronic devices and combination of various functions thereof, signal transmission speed inside of electronic devices or signal transmission speed outside of electronic devices is increasing. Thus, a printed circuit board using an insulator having a lower dielectric constant and a lower coefficient of dielectric loss than the existing insulators is required.
Recently, reflecting these tendencies, there has been an attempt to apply a liquid crystalline polymer (LCP), which has a lower dielectric constant and is less influenced by moisture absorption than the existing polyimide, to a flexible printed circuit board. However, even if the LCP is applied, the dielectric constant of LCP (Dk=2.9) is not significantly different from that of polyimide (Dk=3.2), and thus an improvement degree according to the application is slight. Further, since heat resistance of the LCP is so low that a problem may be caused in a soldering process, and the LCP is thermoplastic, there is a problem in terms of compatibility with a PCB manufacturing process using the existing polyimide in via hole processing using a laser.
Thus, as a solution thereto, there has been an attempt to lower the dielectric constant of a polyimide that is used as an insulator of the existing flexible circuit board. For example, U.S. Pat. No. 4,816,516 describes mixing a polyimide with a fluorine-containing polymer to prepare a molded product. However, the above patent does not relate to a product for an electronic device that requires a low dielectric constant, but relates to a molded product, and practically uses a polyimide having a large coefficient of thermal expansion and a low glass transition temperature. In addition, a polyimide resin should be processed in the form of a thin film so as to be used in a printed circuit board, but the above US patent does not describe a copper foil laminate prepared in the form of a thin film.
Further, U.S. Pat. No. 7,026,032 discloses a method of lowering the dielectric constant of the prepared product by dispersing a fluorine-containing polymer micropowder in a polyimide. The above US patent describes that the fluorine-containing polymer micropowder is distributed a lot on the outer surface compared to the inner core of the insulator. However, as described in the above US patent, since the content of the fluorine-containing polymer is high in the outermost layer of the insulator, moisture penetration and absorption may be decreased by the fluorine-containing polymer on the outer surface, thus decreasing total moisture absorptivity, but a problem that was not seen in the existing flexible copper foil laminate consisting of the polyimide may be generated. For example, the polyimide resin described in the above US patent may have low adhesion to a coverlay or a pre-preg and low adhesion to ACF, the coefficient of thermal expansion (CTE) of the polyimide resin described in the above US patent is too high to be applied in a flexible copper foil laminate, and an excessive amount of fluororesin exists on the surface of the polyimide resin, and thus fluororesin may melt at a temperature of around 380° C. that is applied for a receiving process during a manufacturing process of a PCB, and there is a risk of delamination of the copper foil circuit from the insulator.
Accordingly, there is a demand for development of a material that exhibits a low dielectric constant, and yet has a low coefficient of thermal expansion, high elasticity, and low moisture absorptivity, in order to manufacture a printed circuit board with a low dielectric constant.